A fiber-optic radiation sensor (FORS) for measuring therapeutic radiation is generally composed of an organic scintillator and a plastic optical fiber. Due to such a configuration, upon measuring therapeutic radiation, an organic scintillator having a diameter of 1 mm or less, which is a water or tissue equivalent, is used. Thus, an FORS may reduce a correction task resulting from a difference in material from a phantom, may have high spatial resolution, and may be manufactured as a multi-dimensional sensor. Further, since an optical fiber is used, it is possible to perform long-range measurement in real time without interference from electromagnetic waves.
However, a common disadvantage of measuring devices that use a scintillator is a quenching effect, which may be applied even to fiber-optic radiation sensors.
Generally, if stopping power is low when a charged particle passes through a scintillator, the amount of scintillation occurring in the scintillator is proportional to the amount of energy lost in the charged particle, but if the stopping power is high, the amount of scintillation becomes close to a saturation state.
That is, when stopping power is high, a phenomenon in which the amount of scintillation occurring in the scintillator becomes out of proportion to the amount of lost energy of the charged particle is referred to as a “quenching effect.”
Such an effect dearly appears upon measuring a high-energy proton beam using a scintillator.
Since stopping power at the Bragg peak of a proton beam is very high, a relative dose in the peak section of a proton beam is measured as a value lower than an actual value, upon performing measurement using the scintillator.
Therefore, the measurement of a high-energy proton beam using the scintillator requires a correction task using Birk's formula or the like.
Further, Cerenkov radiation generated in the optical fiber itself, other than the scintillator, due to a direct action between a glass or plastic optical fiber and a charged particle, has been reported as a problem in fiber-optic radiation sensors of existing inventions.
Cerenkov radiation, which denotes conically formed light having a predetermined angle with respect to an incident beam when a charged particle passes through a medium at a speed higher than the speed of light in the medium, is generated by a charged particle having a certain energy (in the case of a plastic optical fiber, electron: 170 keV, proton: 400 MeV) or more.
Therefore, when a dose is measured using a fiber-optic radiation sensor, it is dependent on the traveling direction and the emission intensity of a charged particle due to the emission angle of Cerenkov radiation and the length of an optical fiber to which the radiation is emitted.
However, since such Cerenkov radiation is a signal generated due to an interaction between radiation and a medium, it may be a significant signal upon measuring a relative dose if the emission angle and the length of the optical fiber to which radiation is emitted are fixed.
In particular, when the relative dose of a high-energy proton beam is measured using Cerenkov radiation generated in the optical fiber, a quenching effect attributable to the use of a scintillator may be eliminated, and thus there is an advantage in that a relative dose may be measured without requiring a special correction task.
Cerenkov radiation generated in an optical fiber by a therapeutic proton beam is generated by secondarily or tertiarily generated electrons without being directly generated due to the energy of the proton beam.